The invention relates to a process for oxidizing metal oxides on surfaces wetted by aqueous solutions and more particularly to a process for oxidizing chromiumcontaining radioactive films which tend to form on the surfaces of reactor coolant systems in nuclear power plants. The invention is particularly useful for decontaminating boiling water and pressurized water reactors and thereby reducing the radiation exposure of workers during routine maintenance and operating activities, reactor refueling and plant decommissioning.
As a nuclear reactor generates electric power, the wetted surfaces of the reactor coolant system tend to corrode slightly and form surface oxides of iron, chromium, nickel, cobalt and other metals employed in the system. At least some of the corrosion products (referred to in the nuclear industry as "crud") are transported by the high velocity coolant water to the core region of the reactor vessel and become radioactive. Subsequently, the radioactivate corrosion products are retransported from the core of the reactor vessel and deposit in the balance of the reactor coolant system and thereby increase the radiation fields throughout the plant. In addition, radioactive ions such as cobalt 60 will also deposit on these surface oxides. These radioactive deposits are the principal source of the out-of-core radiation fields and make the greatest contribution to personnel radiation exposure.
Various dilute chemical decontamination processes have been developed for use in boiling water reactors and pressurized water reactors. Typically, these processes dissolve the metal oxides and then recover the dissolved ions and the process chemicals on resin beds and filters. Various permanganate processes have been developed to oxidize such metal oxides as chromium (III) oxide. In alkaline-permanganate processes, coolant water containing potassium permanganate and sodium hydroxide is circulated through the reactor coolant system to oxidize chromium (III) oxide to chromium (VI) oxide, which is soluble in aqueous solutions. After the permanganate-containing water has been circulated for up to several hours, the residual amounts of unreacted permanganate in the coolant water and manganese dioxide formed in the chromium oxide oxidation step are "destroyed" or reduced to manganous ions with oxalic acid. Alternatively, acid permanganate and nitric acid permanganate processes for oxidizing chromium (III) oxide may be employed.
These permanganate chromium oxidation processes are normally combined with other known processes which reduce other metal oxides such as ferric oxide and nickel oxides (e.g.,NiFe.sub.2 O.sub.4) at the surface to acid soluble oxides. In the proprietary Can-Decon and Can-Derem processes, coolant water containing organic acids and typically having a pH of about 2.5-3 is circulated through the reactor coolant system. In the LOMI (Low Oxidation-state Metal Ion) process, coolant water containing vanadous formate and picolinic acid and typically having a pH of about 4-6 is circulated through the reactor coolant system. For a general discussion of these and other processes, M. E. Pick et al., "The Nature Of PWR Stainless Steel And Inconel Oxides In Relation To Decontamination In Permanganate Based (NP and AP) Processes", Water Chemistry Of Nuclear Reactor Systems 3, Vol. 1, British Nuclear Energy Society, London, 1983; T. Suwa, "Development of Chemical Decontamination Process with Sulfuric Acid-Cerium (IV) for Decommissioning", Journal of Nuclear Science and Technology, 23(76), pp 622-632, July 1986 and "Plant Decontamination Methods Review", Electric Power Research Institute, NP-1168, 1981. In practice, a succession of alternating permanganate chromium oxidation steps and nickel and iron reduction steps are performed to dissolve the surface oxides and thereby to decontaminate reactor systems to acceptably low levels.
The unreacted permanganate ions and manganese dioxide which forms in the course of the chromium oxidation step must be destroyed before the following iron and nickel reduction step is employed. Oxalic acid is generally employed for this purpose. However, oxalic acid generates a considerable amount of carbon dioxide in the course of the permanganate destruction process and has been found to be the cause of intergranular stress corrosion cracking in reactor coolant systems. Thus, the use of oxalic acid must be very carefully controlled. Accordingly, in decontamination processes, the permanganate-containing water temperature is lowered from the chromium oxidation temperature of about 90.degree. C (190.degree. F) to below a maximum of about 80.degree. C (175.degree. F). Because reactor coolant systems are particularly sensitive to intergranular stress corrosion cracking caused by solutions containing more than about 1000 ppm oxalic acid at temperatures of 90.degree. C, oxalic acid is added to the reactor coolant systems such that the maximum oxalic acid concentration is less than about 750 ppm.